1. Field of the Invention
The present invention relates to an apparatus for controlling a variable valve actuation mechanism which varies operating characteristics of an engine valve.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 11-082073 discloses an internal combustion engine provided with means for adjusting a phase difference of a camshaft with respect to a crankshaft, and a variable valve timing control apparatus in which a maximum retard position of the camshaft is learned when a target of an advance position amount is set at zero in the camshaft.
Japanese Unexamined Patent Publication No. 2000-8894 discloses an engine provided with a solenoid-driven valve in which the engine valve is withdrawn and driven by a solenoid coil, and a control apparatus in which an output value of a lift sensor is learned as a value corresponding to a reference position when the engine valve is stopped at a moment of starting of the engine.
As described in the foregoing, in the conventional mechanism for variably actuating an engine valve, i.e., the conventional variable valve actuation mechanism, the learning of the reference position is performed on a condition such that the variable valve actuation mechanism is controlled at a reference position based on a request of an engine operating condition.
Accordingly, a problem is encountered in which the condition of the learning is limited to a particular operating state to thereby hardly ensure a sufficient learning frequency.
At this stage, if the variable valve actuation mechanism is forcedly driven at the reference position, restriction of the learning condition will not occur.
However, if the variable valve actuation mechanism is forcedly driven at the reference position, an amount of intake air by the engine changes due to the change in the operating characteristics of the engine valve. Therefore, an engine torque demanded by a driver becomes different from the actual engine torque.
Accordingly, in the conventional variable valve actuation mechanism, it is practically impossible that the learning is performed while the variable valve actuation mechanism is forcedly driven at the reference position.